Abstract:
To protect surface water bodies, final effluents of wastewater treatment works are being
regulated with stricter consents. Phosphorus has been identified as a priority compound
and according to the Water Framework Directive its levels in wastewater effluents need
to be below 1 mg P/L and in some cases as low as 0.1 mg P/L. To meet these consents
efficiently and economically, there are several novel or established tertiary P removal
technologies.
Chemical P removal is a conventionally applied process. Yet, it requires optimisation in
chemical doses and the most appropriate solids liquid separation for tertiary P removal
needs yet to be identified to meet the new stricter consents sustainably. Novel tertiary
P removal technologies such as immobilised algae beads systems or reactive media constructed
wetlands provide a more sustainable approach with no direct use of chemicals on
site and the recycling of materials such as the media in the wetland or through conversion
of the algal biomass to energy. However, these technologies are not yet fully established
and require validation of their viability and cost competitiveness.
In this thesis, tertiary P removal technologies have been evaluated with the aim to resolve
existing bottlenecks that are associated with the implementation of these technologies
when meeting sub 1 mg P/L levels. Three coagulation-based technologies that have not
been operated previously in the UK were assessed on their robustness and resilience under
steady-state and dynamic conditions against a 0.3 mg P/L target. It was found that
ballasted coagulation was the most robust and could consistently deliver effluent concentrations
as low as 0.1 mg P/L. Pile cloth media filtration and ultrafiltration were shown to
be less robust yet effective at reaching 0.3 and 0.5 mg P/L targets, respectively. The importance
of the solid liquid separation step as well as optimisation of dosing and coagulation-
flocculation was highlighted. Further it was found that in a ballasted coagulation system,
weaker and bigger flocs are generated through the addition of polymer which are efficiently
separated through the incorporation of a ballasting agent. Ultimately, guidance on
suitable choice of polymers and their doses was given as anionic polymers at doses as low
as 0.1 mg/L.
From the novel alternatives, a reactive media (steel slag) constructed wetland was operated
at full-scale under real conditions and has reached the highest reported P retention
capacity to date with very low P effluent concentrations (<1 mg/L from an average of
about 8 mg/L) achieved in the first year of operation. During the life cycle of the wetland,
P removal decreased substantially, and it was highlighted that the underlying mechanisms
are more complex than previously assumed. To address the bottleneck of high costs of
beads production in immobilised algae systems, a proof-of-concept has been given where
69.1% alginate recovery was achieved, and algae beads made from recycled alginate were
further reused in P removal trials. Ultimately, a cost reduction of 34% of operational costs
could be achieved.
Finally, the insights were translated into a P removal strategy where the most suitable technologies
are recommended for differently scaled wastewater treatment works (WWTW)
and different effluent P targets based on their performance, costs and sustainability. For
large WWTW, ballasted coagulation appeared to be the most suitable technology while
for small WWTW pile cloth media filtration is recommended. Based on this research,
ultrafiltration cannot be recommended for tertiary P removal. The novel technologies
were highlighted as more sustainable options for small WWTW which still need further
understanding and development.